The background description for this invention is presented only by way of example with reference to after develop inspection (ADI) for macro-size surface defects on a patterned or an unpatterned semiconductor wafer specimen.
The objective of macro-size defect inspection is to ensure that the wafer is free from yield-limiting large scale (i.e., greater than about 25 micron minimum dimension range) defects, such as incomplete or extra photoresist coverage, regions of defocus ("hot spots"), scratches, pattern blemishes, particles, and nonuniform or incomplete edge bead removal (EBR). With the exception of EBR, such defects appear on either a lot-to-lot (systematic) or a wafer-to-wafer (random) basis; therefore, a typical strategy is to inspect some or all wafers from each lot of wafers.
One popular way of performing ADI for macro-size defects entails placing the wafer on a semiautomatic tilt table and rotating the wafer through various angles under a bright light. U.S. Pat. No. 5,096,291 describes one type of semiautomatic tilt table that rotates a specimen about a central axis while positioning the specimen in different inclinations relative to a plane normal to the central axis. An operator visually inspects the wafer as it rotates and changes inclination and renders decisions about the presence or absence of defects. Wafers are either passed or rejected, based upon the operator's judgment. Defects very large in size in comparison to the minimum device design geometry dictate the degree of detection sensitivity required in connection with ADI for macro-size defects. This function is currently performed with the human eye aided by high intensity light in a dark field or bright field configuration. FIG. 1 shows five types of defects typically found during ADI for macro-size defects on a surface of a semiconductor wafer specimen.
With reference to FIG. 1, defect location A represents incomplete photoresist coverage; defect location B represents a surface scratch; defect location C represents extra deposited photoresist; defect location D represents a "hot spot"; and defect location E represents nonuniform edge bead removal. The term "hot spot" refers to a photoresist exposure anomaly caused by a depth of focus limitation or nonuniformity generally resulting from planar nonuniformity of the wafer at the time of exposure, the presence of foreign material on the backside of the wafer or on the wafer support system, or to a photolithography equipment problem or design constraint. The foreign material effectively deforms the wafer, which as a consequence presents a nonuniform focal surface during photolithography exposure. The existence of nonuniform focus during the photolithography process manifests itself as an unwanted pattern feature change.
Each of the defects identified above has a characteristic signature that manifests itself under either dark field or bright field illumination. Table 1 presents each such defect together with its characteristic signature produced under the appropriate illumination field.
TABLE 1 ______________________________________ Characteristic Signature Under Bright Field (BF) and Dark Field (DF) Illumination at Various Defect Type Wafer Inclination Angles ______________________________________ Scratches DF: Bright line on dark background Incomplete photoresist BF: Thin film interference effects coverage Extra photoresist BF: Thin film interference effects Large defocus DF: Dim or bright pattern compared to neighboring die Nonuniform edge BF: Nonuniform photoresist edge detected at the bead removal edge of the wafer ______________________________________
Other defects or anomalies and distinguishing features having characteristic signatures under bright field and dark field illumination at various wafer inclination angles include no or partial exposure, large line width variations, overexposure, large particles, comets, striations, no photoresist deposited, underdeveloped photoresist, double exposure, development spots, and double development.
The high monetary value of each wafer makes this inspection strategy viable because the wafers generally can be reworked and abnormal process excursions can be readily detected and corrected. Lithography and automation trends in the industry are making ADI more critical and operator involvement less useful; therefore, an automated solution is needed.